One of the targets in future wireless communication networks, e.g. the LTE (Long Term Evolution), is an Uplink (UL) peak data rate of 500 Mb/s. To achieve this data rate the UL transmission scheme in LTE must be extended to support wider bandwidth and single user spatial multiplexing (SU-MIMO).
In LTE, spatial multiplexing and transmit diversity are specified multi-antenna transmissions schemes. In spatial multiplexing, incoming code blocks are possible split and then mapped to different layers. If diversity is used, the code blocks are copied onto several layers as the same information is to be sent over many channels. The precoder will then take one symbol from each layer and perform a mapping in the frequency domain.
The objective is to achieve a precoding, where the precoded layers are emitted from the transmit antennas with appropriate weighting per each antenna such that the link throughput is maximized at the receiver output.
The precoding in SU-MIMO of LTE can either be based on a predefined codebook or the precoder can take any value referred to as non-codebook based precoding.
In the codebook based precoding, a precoder is chosen out of a predefined set—the codebook—and applied to the Physical Uplink SHared Channel (PUSCH) and possible to the reference signals. Typically the index of the chosen codebook entry—the actual precoder—is known to the receiver. In the context of codebook based UL MIMO, it is expected that the eNodeB will not only determine modulation and coding scheme (MCS) as in Release 8 of the 3GPP-specifications for E-UTRA but also the precoding weights. Transmission parameters comprising MCS and precoding weights (typically expressed as index in code book) are then signaled as part of the downlink control channel information carrying the UL grant to the transmitting UE.
In case that also the reference signals are precoded with the same precoder it is actually not needed that the receiver is aware of the applied precoder.
In non-codebook based precoding the precoder can take any value, and is likely determined by the transmitter. Since the precoder can take any value and no index is signaled to the receiver, the receiver can not make any assumptions on the precoder value. To enable demodulation of PUSCH at the receiver also the reference signals are precoded. Non-codebook based precoding could be illustrated as follows: The terminal can analyze DownLink (DL) channel measurements and calculate the “perfect” precoder for the current DL channel, especially for TDD. Because of channel reciprocity in TDD, UL and DL channel are the same and thus the precoder based on DL measurements matches also the UL and can be applied. The precoder is used directly without quantizing to the nearest codebook entry and thus matches the channel better than a codebook entry would do.
Further, the precoder may be frequency selective or constant over a wider frequency band.
Hence, a frequency-selective precoder changes over frequency. A typical granularity over which a frequency-selective precoder is constant could be for example one or several resource blocks. But also precoders varying with every subcarrier are possible. In case of wideband precoding a single precoder is used across the allocated bandwidth.
Together with codebook-based precoding a wideband precoder has the advantage that signaling is reduced since only a single precoder index needs to be signaled for the whole bandwidth. However, using a single precoder over a wide bandwidth leads to sub-optimal performance.
For non-codebook based precoding no precoder indices needs to be signaled and thus a more frequency-selective precoder can be used.
In LTE so far it has only been specified UL transmissions from a single antenna port at a time. What however is possible with the current framework is simultaneous transmission of multiple terminals on the same time-frequency resources, a.k.a. Multi-User MIMO (MU-MIMO) or spatial domain multiple access. From a terminal perspective this transmission mode is the same as the standard single-antenna port UL transmission, only the base station is required to implement a special receiver for this transmission mode.
In order to estimate the propagation channels between the various terminals and the base station mutually orthogonal reference signals are needed. In LTE these reference signals are defined in frequency-domain which is further explained in 3GPP TS36.211,r(α)(n)=ejαn r(n),n=0,1, . . . ,MSCRS−1,  (1)with MSCRS=mNSCRB the number of allocated subcarriers. The sequence r(α)(n) is directly mapped onto the allocated subcarriers without DFT precoding. Multiplication of the base sequence r(n) with the complex exponential function in frequency domain results in a cyclic shift of
      N    12    ⁢      n    CS  samples of the time-domain base sequence, α=2πnCS/12 (N is the size of the IDFT used in the modulator). The base sequence r(n) is cell specific. It has constant magnitude resulting that the time-domain sequence (application of IDFT to r(n)) has a perfect periodic autocorrelation function. Because of this property all sequences derived via (1) are orthogonal to each other.
Equation (1) together with the definition of a reveals that in total 12 different cyclic shifts (nCS=0, 1, . . . , 11) exist resulting each in a different sequence r(α)(n). However, in LTE only a 3-bit field is specified to signal nCS (and thus α) resulting in 8 possible cyclic shifts and thus 8 orthogonal reference signals. Table 1 below shows the table defined in 3GPP TS 36.211 specifying the cyclic shifts. The signaled parameter nDMRS(2) determines the cyclic shift value according tonCS=(nDMRS(1)+nDMRS(2)+nPRBS)mod 12,  (2)with nDMRS(1) and nPRBS being cell specific parameter. nDMRS(2) is signaled to the terminal as part of the UL scheduling grant message. Table 1 below specifies the cyclic shifts.
Cyclic Shift FieldinDCI format 0 [3]nDMRS(2)000000120103011410061018110911110
For SU-MIMO orthogonal reference signals for each transmission layer or transmission antenna are needed. One possibility is to assign sequences with different cyclic shifts to the different layers or antennas.